If you can't finish the final set of reps, that is alright as several trainers suggest that taking your muscles to the point of failure will build them the best. It is also more than appropriate to use lighter weights and be able to finish all the reps. Taking muscles to failure will build reps while using lighter weights will tone. Either way, be sure to focus on form and exercising the muscle from both angles. This means that you do the repetitions slowly both up and down.